1. Field of the Invention
The invention relates to an improved method and apparatus for repairing or reworking complex multilayer printed circuit board interconnections.
2. Description of the Prior Art
It is known to utilize printed circuit boards to make electrical interconnections between leads of electrical components mounted thereon. Printed circuit boards characteristically involve one or more discrete layers of insulating material upon which patterns of electrical conductors are formed in conjunction with a predetermined array of holes. The electrical conductors are referred to as foil. The layers are stacked, bonded, and the hole patterns formed and plated-through with electrical conductive materials. Characteristically, plated-through holes are uniform in diameter, and are often referred to as "barrels". Selective interconnections result in selected wiring networks, or nets, being formed with various ones of the electrical conductors on different layers being interconnected through the plated-through holes. At the mounting surfaces, it is known to provide additional electrically conductive material in electrical contact with the plated-through holes for the purposes of providing an expanded pad area for making interconnection with the component contacts, terminals, or leads.
It has been the usual practice to have the diameter of the plated-through holes uniform throughout the thickness of the multilayer printed circuit board. With the advent-of continued reduction in size of the electronic components, the spacing of the component connections, and the associated array of holes, has been ever-reducing. At the same time, the complexity of the interconnections that are to be made by the multilayer printed circuit boards has increased. This increase in the number of interconnections results in the requirement of providing more conductive routing paths on the various layers, where these routing paths must be constructed in ever-decreasing dimensions. The loss of routing area resulting from the reduction in pin spacing in the grid array of plated-through holes has lead to the requirement of adding additional layers to the multilayer printed circuit board assemblies, with the attendant increase in cost of manufacture.
The increase in the number of layers leads to additional problems of registration of the layers. Registration is positioning and placement of the grid array of holes of the layers with respect to the other layers in the assembly. Poor registration can result in reduced reliability of the formation of the electrical interconnections to the respective plated-through holes in the grid array. Reduction of the hole diameters will result in more area per layer for routing paths. Greatly increasing the number of layers and the number of plated-through holes on a printed circuit board assembly renders each such printed circuit board that is successfully constructed and tested, a valuable commodity.
It is known to utilize so-called surface mount components, where relatively short component leads are affixed to surface pads on the multilayer printed circuit boards. Characteristically, they are affixed through a solder process. It is also known to provide long-lead interconnection of components to the multilayer printed circuit board, wherein the component leads are formed and inserted in respectively associated ones of the plated-through holes. After such insertion, the leads are soldered in place. Both forms of interconnect have problems attendant to the manufacturing processes. The surface mount has the problem of location of all of the terminals of a component over the associated interconnection pads during the soldering process. Any misalignment or misplacement can result in missing or marginal solder interconnections. The insertion of the long-lead electrical components can result in bent leads not properly being inserted through the depth of the plated-through holes, thereby either slowing the manufacturing process if detected, or resulting in a defective assembly if not detected at the time of component insertion.
It is also well-known in the design of the electrical interconnections to be made on the printed circuit board assembly, that design problems can result in wiring network layouts that must be altered or reworked to form correct wiring network interconnections. Further, it is known that in the fabrication of multilayer printed circuit boards it can occur, for various manufacturing reasons, that one or more layers may have electrical conductors improperly electrically shorted to a plated-through hole. Unless such a shorted condition can be repaired, the entire assembly has to be scrapped. Various types of rework and repair techniques have been developed, but such known repair and rework techniques are complex; and if not accomplished with skill and precision, can result in further damage to the printed circuit board and failure to remedy the problem. As the printed circuit boards become larger there is an increase in the number of plated-through holes of smaller diameter, and more layers, are utilized, it becomes more and more necessary to have an effective and efficient way to rework incorrect or defect wiring nets to save the boards.
In prior art multilayer boards it was common to have component leads extend through the thickness of the board and to be soldered within the plated-through holes. Structures that received component leads were generally of a larger diameter and the printed circuit boards were of a thickness that allowed the leads to pass through the board. To rework this type of board, it was common to drill out the hole and insert an insulating sleeve or fill it with nonconductive material such as epoxy. The fill material was drilled to receive a component lead. Clearly the drilled hole size or the sleeve opening would require sufficient size to accept the component pin after the repair process. As board hole sizes decreased and board thickness increased, this rework process becomes difficult, if not impossible. This prior art rework process had the further disadvantage that the component leads often were trimmed to the thickness of the printed circuit board. Obviously this did not provide any terminal length to which a rework wire could be coupled on the solder side of the board. To alleviate this deficiency, it was necessary to attach conductive lead extenders so that the external rework wires could be attached. These rework processes cannot be efficiently and reliably accomplished on thick printed circuit boards having high-aspect ratio plated-through holes.